(a) Field of the Invention
The present invention relates to a brushless direct current (BLDC) motor. More specifically, the present invention relates to an apparatus and method for determining a commutation time of a sensorless BLDC motor.
(b) Description of the Related Art
A BLDC motor driving circuit can effectively drive a motor when a continuous rotating magnetic field of the BLDC motor is formed. In order to form the continuous rotating magnetic field, commutation of each phase must be performed at a proper time. In order to perform a proper commutation, the position of the rotor must be detected. Therefore, devices to detect the rotor are needed, and the devices to detect the position of the rotor include Hall sensors and resolvers. However, when Hall sensors or resolvers are used, the circuit becomes complicated. Hence, in order to ameliorate the inconveniences caused by the addition of Hall sensors, a sensorless BLDC motor has been developed.
The sensorless BLDC detects the position of the rotor using the back electromotive force (BEMF) caused by rotation of the motor.
A proper commutation time of the sensorless BLDC motor will be described.
When the BLDC motor has a large BEMF, the torque of the motor becomes great. Therefore, in order to most effectively drive the motor, a current must be supplied to a coil of a phase having the greatest BEMF. Hence, commutation must be performed at a point delayed by about 30.degree. from a zero cross point of the BEMF distribution curve of each phase in a three phase BLDC motor.
Therefore, it is important to detect the point delayed by 30.degree. from the zero cross point of the BEMF distribution curve in the sensorless BLDC motor so as to effectively drive the motor.
In a conventional method, an R-C combination circuit is used to detect this commutation point.
A conventional apparatus for detecting the commutation point will now be described.
FIG. 1 is a schematic diagram of a conventional apparatus for detecting the commutation point. FIGS. 2(a) and (b) are graphs representing the operations for detecting the commutation point with the apparatus of FIG. 1.
As represented in FIG. 1, a resistor R and a capacitor C are coupled to a stator coil of a phase of the motor in the conventional commutation point detecting apparatus. The voltage at the capacitor C is supplied to a positive terminal of a comparator COMP1, and the voltage at a neutral point is supplied to a negative terminal of the comparator COMP1.
As can be inferred from the drawing, the phase of the voltage that the capacitor C supplies to the positive terminal of the comparator COMP1 is delayed compared to the phase voltage of the motor because of the capacitance of capacitor C. When the values of the capacitance of capacitor C and the resistance of resistor R are adjusted, the time at which the voltage at the capacitor C reaches the voltage of the neutral point can be used as the time to perform commutation. This method is advantageous when the rapid changes of the motor phase voltage causes the capacitor terminal voltage to also follow identical changing patterns. However, since the conventional commutation time detecting apparatus has fixed values of the resistance of resistor R and of the capacitance of capacitor C, an accurate commutation time cannot be determined except for a specific rotation speed.
In order to overcome this problem, values of the resistor and the capacitor appropriate to the rotation speed in a uniform speed drive mode are used. However, when the rotation speed is changed, as in the case of acceleration, efficiency is reduced and use of power is increased, so that the time to reach the uniform speed is increased.